The present invention relates to an image forming apparatus and, more particularly, to the structure of a sheet feeder installed in an image forming apparatus.
A facsimile machine, laser printer or similar image forming apparatus is often constructed to produce a reproduction by transferring a visible image formed on a photoconductive element to a plain paper or similar sheet material. A prerequisite with such image transfer is, therefore, that a paper sheet be accurately transported into register with the image on the photoconductive element. A sheet feeder is generally installed in this kind of apparatus for implementing such a manner of sheet feed. The sheet feeder may be of the type including an elevatable table which is loaded with a stack of paper sheets, and pull-out rollers engageable with the paper sheet located at the top of the stack. A guide member is provided at at least one side edge of the table which extends in the widthwise direction of the paper sheets. The guide member is movable along the width of the table to regulate the side edges of the sheet stack on the table, whereby the paper sheets are fed one by one while being protected against skew.
In the sheet feeder of the type having a guide member as stated above, paper sheets are transported, i.e., they are stacked on the table by using either one of two different reference positions which are the center and one side. Specifically, paper sheets are stacked on the table with the intermediate between opposite widthwise edges thereof, or center, being positioned at the widthwise center of the table, or with their one widthwise edge being positioned on the table with no regard to the sheet size. In the center reference scheme, the feed rollers and the paper sheets are held in a constant relationship. This is successful in freeing the rollers of the transport mechanism from uneven wear and preventing the transporting force from becoming irregular in the widthwise direction. However, documents have to be set in matching relation to the reference for the paper sheets by troublesome manipulations. In addition, it is difficult to remove a jamming sheet. In contrast, the one side reference scheme promotes easy document setting and allows a jamming sheet to be removed with ease, because it uses a fixed reference position with no regard to the sheet size. A problem with this alternative scheme, however, is that the pull-out rollers contact only one side of the paper sheet at all times and therefore suffer from uneven wear, degrading the accuracy of paper transport.